JP2014052349A - Method for detecting low frequency signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for detecting a low frequency signal, which eliminates electromagnetic wave noise at low frequencies picked up from an ambient environment and a power line to detect a low frequency signal when detecting the low frequency signal by a sensor coil.SOLUTION: Two sensor coils 5 and 6 are arranged so that their rear surfaces are matched with each other, and a difference between their detection signals is obtained and is subjected to signal processing, whereby electromagnetic wave noise at low frequencies is eliminated. A magnetostatic field is generated by a magnet booster 11, and the sensor coils 5 and 6 are arranged within the magnetostatic field, and variation or disturbance of the magnetostatic field is detected by the sensor coils 5 and 6. A signal processing unit amplifies and rectifies signals detected by the sensor coils 5 and 6. The rectified signal of the sensor coil 6 has the phase inverted and is added to the rectified signal of the sensor coil 5, whereby noise at low frequencies is eliminated to output a detection signal.

Description

  The present invention relates to a low frequency signal detection method for detecting a low frequency signal with a sensor coil. More specifically, the present invention relates to a low-frequency signal detection method for detecting a low-frequency signal by removing a low-frequency noise signal included in a sensor coil when detecting a magnetic field change or a low-frequency electromagnetic wave.

  The sensor coil is used in various fields including a metal detection device. A typical example is detection of metallic foreign matter mixed in a product in the product manufacturing process. Specifically, in the manufacturing process of foods, pharmaceuticals, etc., some containers, blades, sieves, etc., such as conveyors, washing machines, agitators, cutting machines, kneaders, steamers, etc. are subject to wear, metal fatigue, etc. In some cases, fragments due to peeling, breaking, peeling, shaving, chipping, etc., enter the product as foreign matter. These foreign substances are detected by a detecting means using a sensor coil and excluded from the product.

  The sensor coil detects a change in magnetic field or a low frequency electromagnetic wave. As described above, conventionally, when a magnetic field or a low-frequency electromagnetic wave is detected by the sensor coil, low-frequency electromagnetic noise is generated around the sensor coil and detected together with the detection signal. Such electromagnetic noise is suddenly generated around the detection means, and examples of the generation source thereof include an electric motor, an electric cable, a solenoid valve, a pump, and an air conditioner. In particular, when the detection means is a production site, there are a lot of such electromagnetic noises, and if there is something that can identify the generation source, it may not be possible.

  Further, when the ferromagnetic material crosses or links the magnetic field lines around the detection means, this becomes a source of electromagnetic noise. Such a ferromagnetic material is rarely intentionally used on the production site, but depending on the case, an iron tool, a blade, or the like can be used. Furthermore, when a power cable is laid around the detection means, a large magnetic field is generated from the power cable every time a large current flows through the power cable, and becomes a source of electromagnetic noise. That is, the magnetic field around the sensor coil is disturbed.

  Examples of the large current include large voltage fluctuations such as starting or stopping of the power pump, an instantaneous power failure due to lightning, and turning on / off of a large load. These electromagnetic noises disturb the magnetic field around the detection means, which becomes electromagnetic induction noise in the sensor coil and is superimposed on the detection signal. As an application of the sensor coil, the inventors of the present invention have proposed the inventions described in Patent Documents 1 to 3. The devices described in Patent Documents 1 to 3 detect metal foreign objects in an object to be detected such as food, and succeeded in detecting a minute magnetic body using a sensor coil.

  Specifically, Patent Document 1 discloses a metal foreign object detection method and a metal foreign object detection device that detect a metal foreign object mixed in an object to be detected. According to the present invention, not only the metal foreign matter such as stainless steel mixed in the detected object is detected, but also the metal mixed in the detected object such as food, medicine, industrial material wrapped in conductive packaging material. Foreign objects can also be detected.

  This metal foreign object detection device of Patent Document 1 detects a magnetic field detected from a metal foreign object in response to a minute magnetic field by generating a minute magnetic field by a detection unit having one coil having a configuration in which a conductor is wound around a core. It detects as a voltage or a detection current and outputs a detection signal. The minute magnetic field is a voltage applied to the coil at a frequency of several hundreds of Hz to several tens of kHz, or a direct current, and the current supplied is minute, and within the magnetization (BH) characteristics of the core constituting the coil. The magnetic flux density (B) and the magnetic field (H) representing the magnetization characteristics are used in a non-linear portion where the values are very small near zero.

  When the metal foreign object passes in the vicinity of the coil, the formation of magnetic force lines interlinking with the coil is disturbed, and the amplitude, phase, and frequency of the signal voltage are changed, thereby detecting the metal foreign object. This apparatus has an excellent sensitivity capable of detecting metallic foreign objects of 1 mm or less. Further, it is possible to detect an elongated metal object such as a needle in an aluminum package and an antioxidant made of metal powder. The metal detector sensor disclosed in Patent Document 3 is a metal detector sensor for detecting a metal foreign object in an object to be detected. The sensor is arranged in contact with a core and generates magnetic lines of force due to a static magnetic field. And a magnet for magnetizing the metallic foreign matter.

  In Patent Document 3, an alternating magnetic field is generated when an AC voltage is applied to two pairs of sensor coils in which a copper wire coil is wound around an E-type iron core. At this time, two pairs of sensor coils are connected to form a balanced or unbalanced bridge circuit. As long as the alternating magnetic field is not changed, the output current of the bridge circuit is constant. When an object to be inspected containing a magnetic substance or a magnetized metallic foreign object passes between the upper and lower sides of two pairs of sensor coils generating an alternating magnetic field, the form (formation) of the magnetic field lines of the alternating magnetic field is disturbed.

  At this time, the current flowing through the sensor coil changes, and the output voltage of the balanced or unbalanced bridge circuit changes. Metal foreign matter can be detected by the change in the output signal voltage. In the metal detector disclosed in Patent Document 4, the magnet of the magnet booster is arranged in the vicinity of the sensor coil, or the magnet is directly fixed to the sensor to form an unequal static magnetic field linked to the sensor coil. When the object to be detected crosses this unequal magnetic field, the metal foreign object is temporarily magnetized, and at the same time, the magnetic field generated from the magnetized object to be detected disturbs the magnetic field linked to the sensor coil.

  The sensor coil sends out the disturbance of the magnetic field as a detection signal. As shown in FIG. 6, the conventional metal detector inspects the inspection object 2 twice using the detection sensors 70 and 71. This is for reliably detecting and eliminating noise such as electromagnetic noise as described above. The detection sensors 70 and 71 use four of the two pairs of sensor coils 5 each having two upper and lower portions. The detection sensors 70 and 71 are set apart from each other by an L distance, for example, several tens of centimeters.

  When the detection sensor 70 and the detection sensor 71 output the same detection signal at the same time, the detection sensor 70 and the detection sensor 71 determine that it is not foreign matter but noise, and finally output a good product signal. Therefore, unless the inspection object 2 being inspected passes through the detection sensor 71, the next inspection object 2 cannot be conveyed, and the processing speed of the production line is limited. The reason for this is that when two or more inspection objects 2 mixed with foreign matter are transported in succession, there is a region where both detection sensors 70 and 71 generate a signal indicating foreign matter at the same time. It is because there is not.

  In general, in a manufacturing environment such as a food factory, a large number of devices such as motors and electromagnetic valves that induce disturbance of the surrounding magnetic field are used. Sensor coil-type metal detector or metal detector has the disadvantage that the sensor coil accidentally picks up low-frequency electromagnetic noise generated in the vicinity of the sensor coil, which causes false detection and outputs a signal with foreign matter have. As countermeasures against this erroneous detection, various countermeasures are taken against the sensor coil and the source of electromagnetic noise. For example, when the source of electromagnetic noise can be identified, the low frequency electromagnetic noise picked up by the sensor coil is reduced by moving the source as far as possible from the metal detector.

  Further, when the source of electromagnetic noise can be identified, the source is removed or stopped functioning. Furthermore, when the electromagnetic noise generation source has directivity (physical direction or the like), the radiation angle of the electromagnetic noise is changed so that the sensor coil is not present. Further, the electromagnetic noise source is covered with an electromagnetic shield, or the electromagnetic noise source is placed in an electromagnetic wave prevention case.

Japanese Patent No. 3857271 JP 2005-188985 A International Publication WO2003 / 027659 Japanese Patent No. 3875161

  As described above, the electromagnetic noise is detected simultaneously with the detection signal by the sensor coil, and the elimination thereof is important. Conventionally, when a low frequency or high frequency excitation frequency is determined and the detection target signal is generated or displaced depending on the excitation frequency, for example, a lock-in amplifier having a very large dynamic reserve of 120 dB or more is used. There is a signal extraction method. However, this signal extraction method has a limit in the level of a signal that can be detected.

  In particular, if there is electromagnetic noise having the same frequency band as the detection target electromagnetic wave signal and a magnitude of several tens to several hundreds of times from the detection target electromagnetic wave signal, it is very difficult to remove it. As described above, when a sensor coil for inspecting a product foreign matter is used at a production factory, the large electromagnetic noise is generated and detected even if rare from the production equipment, and the reliability of the foreign matter inspection is impaired. Sometimes. Even if the above-mentioned various measures are taken, the electromagnetic noise cannot be reduced or eliminated, or the electromagnetic noise source is the main equipment of the production line, or is separated, repositioned, stopped or removed on the equipment. There are cases where things cannot be done.

  As a countermeasure in such an unavoidable case, the countermeasure may be taken by reducing the detection sensitivity of the sensor coil or changing the design of the line if possible. As described above, the conventional metal detector is easily affected by the source of electromagnetic noise, and the countermeasures are troublesome. Under such circumstances, it is desired to detect a low-frequency signal with a sensor coil without being affected by electromagnetic noise or minimizing it.

The present invention has been made based on the technical background as described above, and achieves the following objects.
An object of the present invention is to provide a low frequency signal detection method for detecting a low frequency signal by erasing low frequency electromagnetic noise picked up from an ambient environment or a power line when detecting a low frequency signal by a sensor coil. To do.

  Another object of the present invention is to provide a low-frequency signal detection method for detecting a low-frequency signal by removing a low-frequency noise signal contained therein when detecting a change in a magnetic field or a low-frequency electromagnetic wave with a sensor coil. provide.

  Still another object of the present invention is to detect a low frequency signal from a signal including low frequency electromagnetic noise around a production line included in the sensor coil when a change in magnetic field or low frequency electromagnetic wave is detected by a sensor coil. A method for detecting a low frequency signal is provided.

In order to achieve the above object, the present invention employs the following means.
The low-frequency signal detection method of the present invention is a low-frequency signal detection method for obtaining a detection signal by removing a noise signal from a detection signal in which a magnetic field or electromagnetic wave is detected by a detection means,
The detecting means includes a magnetic field generating means for generating a magnetic field, a first sensor coil arranged in the magnetic field, and a second sensor arranged in the magnetic field in the vicinity of the first sensor coil. Consisting of coils,
Each of the first sensor coil and the second sensor coil is for detecting a change in the surrounding magnetic field or a low-frequency electromagnetic wave, and has a configuration in which a conductor is wound around a core, The detection is performed using a non-linear portion in which the magnetic flux density (B) and the magnetic field (H) of the magnetization (BH) characteristic are very small values near 0,
A difference signal indicating a difference between the first detection signal detected by the first sensor coil and the second detection signal detected by the second sensor coil is obtained by the signal processing means, and the low frequency noise signal is removed.
When the difference signal is greater than or equal to a predetermined amplitude value, the signal processing means outputs the detection signal indicating the change magnetic field in which the magnetic field changes or the low frequency electromagnetic wave that is not derived from the low frequency noise signal. Features.

The following describes the invention in detail.
In the present invention, the low frequency signal or the low frequency electromagnetic wave means a signal or electromagnetic wave having a frequency of 5 Hz or more and 10 Hz or less. The low-frequency signal detection method of the present invention is an invention for detecting a detection signal by detecting a change in a surrounding magnetic field, a low-frequency electromagnetic wave, etc., by detecting means including two sensor coils, and removing low-frequency electromagnetic noise. It is. A detection means consists of the 1st sensor coil arrange | positioned in the formed magnetic field, and the 2nd sensor coil arrange | positioned in the vicinity.

  The first sensor coil and the second sensor coil have a configuration in which a lead wire is wound around the core, and the magnetic flux density (B) and magnetic field (H) of the magnetization (BH) characteristic of the core are very small values near zero. Detection is performed using a certain nonlinear part. The low-frequency electromagnetic noise is removed by signal processing of the first detection signal detected by the first sensor coil and the second detection signal detected by the second sensor coil by the signal processing means. Specifically, a difference signal indicating the difference between the first detection signal and the second detection signal is obtained by the signal processing means, and the low frequency electromagnetic wave noise is removed.

  The signal processing means determines the difference signal and determines whether the signal indicates a change magnetic field or a low-frequency electromagnetic wave that is a detection target. When the difference signal is greater than or equal to a predetermined amplitude value, the signal processing means outputs a detection signal indicating a changing magnetic field or a low-frequency electromagnetic wave, which is a signal not derived from the low-frequency electromagnetic wave noise. The detecting means generates a magnetic field on the first sensor coil and the second sensor coil, and detects the disturbance of the magnetic field. Magnetic field disturbance occurs when a magnetic material moves or passes through the magnetic field.

  The generation of the magnetic field can be formed by exciting the first sensor coil and the second sensor coil. In this excitation, a low-frequency current from DC to 10 kHz or less is passed through the first sensor coil and the second sensor coil. The magnetic field can be generated by arranging a magnet booster in the vicinity of the first sensor coil and the second sensor coil and using the magnetic field formed by the magnet booster. Furthermore, the magnetic field can be generated by combining the excitation of the first sensor coil and the second sensor coil and the magnet booster.

  Thus, when the generated magnetic field is disturbed or fluctuates, this is detected by the sensor coil. The first sensor coil and the second sensor coil have a surface formed by winding the conductive wire around a core. Since this surface is a surface through which magnetic flux lines of magnetic field enter and exit, it can also be called a detection surface. The surface opposite the front surface is called the back surface. The back surface is the surface of the core where magnetic flux lines do not enter and exit. This change in the magnetic flux lines results in a changing magnetic field detected by the first sensor coil and the second sensor coil.

  The surface of the first sensor coil is arranged toward the changing magnetic field to be detected and the source of the low-frequency electromagnetic wave. At least in this way, the change magnetic field and low frequency electromagnetic wave to be detected can be efficiently detected by the first sensor coil. The surface of the second sensor coil is arranged in the direction opposite to the surface of the first sensor coil. The opposite direction is a direction in which the vertical lines of the surface of the second sensor coil and the surface of the first sensor coil are different in direction by 180 degrees and are directed in opposite directions.

  In other words, the back surface of the second sensor coil and the back surface of the first sensor coil are arranged in contact with or opposed to each other. In such an arrangement, it is difficult for the second sensor coil to detect the changing magnetic field and the low-frequency electromagnetic wave to be detected. At least in such an arrangement, the change magnetic field and the low frequency electromagnetic wave to be detected are detected by the first sensor coil from the second sensor coil. Further, the surface of the second sensor coil may be arranged toward a noise signal generation source that is a generation source of the noise signal.

  Since the noise signal generation source is practically a certain distance away from the first sensor coil and the second sensor coil, the first sensor coil and the second sensor coil detect the same noise signal. In other words, the noise signal generated from the noise signal generation source is similarly detected by the first sensor coil and the second sensor coil. The above-mentioned changing magnetic field occurs when a magnetic field generation source such as a magnetic body moves in a static magnetic field. When the magnetic body moves, the magnetic flux affects the magnetic flux of the first sensor coil, which changes the current flowing through the first sensor coil.

  This change in current is output as a detection signal. As an example of the low frequency noise signal, electromagnetic noise generated from an electric circuit, an electric device, an electronic device, a moving or rotating permanent magnet, a power line, or the like when viewed from the production site. The signal processing means rectifies the first detection signal and the second detection signal. The rectified second detection signal is phase-inverted and added to the rectified first detection signal to obtain a difference signal. By this phase inversion and addition, the low frequency noise signal is eliminated, and only the detection signal remains and is output as a difference signal.

  The first sensor coil and the second sensor coil may be disposed with their surfaces facing away from each other and their backs together. Of course, the first sensor coil and the second sensor coil can be arranged so that the longitudinal axis thereof has a predetermined angle, but this is fine-tuned while confirming the noise signal erasure sensitivity. Is preferably determined. In general, it is preferable that the predetermined angle is set to 0 and the rear surfaces are arranged in parallel. As described above, when the change magnetic field or low-frequency electromagnetic wave to be detected is detected by the detection means, the surrounding electromagnetic noise detected at the same time is removed by the signal processing means.

  In particular, it is more effective than the conventional method when the intensity of the electromagnetic wave noise is larger than the intensity of the changing magnetic field or the low-frequency electromagnetic wave to be detected. The present invention extracts a low-frequency weak detection target signal from a low-frequency large noise signal. In other words, even if the S / N ratio (signal-to-noise ratio) between the low frequency weak detection target signal and the low frequency large noise signal is very small, the changing magnetic field or low frequency electromagnetic wave that is the detection target is It can be detected from a large noise signal of a large low frequency.

  In the present invention, there is no standard frequency or reference frequency band for separation / extraction as in the prior art. The magnetic field generation source can be a minute magnetic material. The low frequency noise signal can be any low frequency noise. The signal processing means may have a signal processing unit such as a phase transfer unit, an LPF unit, a notch filter unit, a peak hold unit, and a reset unit in order to detect more efficiently. The phase transfer unit is for finely adjusting the phase of the detected first detection signal and second detection signal.

  Even if it says that the arrangement | positioning of a 1st sensor coil and a 2nd sensor coil is near, it becomes a spatially separate place. Thereby, the noise signal generated from the noise signal generation source is detected at different phases by the first sensor coil and the second sensor coil. Therefore, the phase transfer unit is for finely adjusting the phase to be the same. The phase transfer unit is realized by an arbitrary known circuit. When the intensity of the noise signal is larger than the intensity of the changing magnetic field and the low frequency electromagnetic wave, the present invention can detect the detection target signal effectively.

  The change magnetic field and low-frequency electromagnetic wave to be detected can be efficiently detected by the first sensor coil. The first sensor coil is excited at a low frequency from DC to 10 kHz or less. In practice, the magnetic material that moves or passes near the first sensor coil is actually transported by the transport means. For example, the magnetic material is contained in the object to be inspected and is conveyed by a conveying means such as a belt conveyor. In this case, the first sensor coil and the second sensor coil do not move but are fixed, and the magnetic body moves at the conveyance speed of the conveyance means.

  From the actual example of the belt conveyor of the current product production line, the conveyance speed is constant and often falls within the range of 0.1 m / min to 100 m / min. And the magnetic body conveyed at such a conveyance speed has a frequency of a signal detected by the first sensor coil of 0.1 Hz to 25 Hz, at most about 40 Hz. However, the detected signal differs depending on the conveyance speed of the magnetic body, the physical detection width of the first sensor coil, the size of the magnetic body, the difference between the magnetic body and the relative permeability, and the like. The time constant, frequency response, etc. of the detection circuit also affect this and are difficult to calculate uniquely.

According to the low-frequency signal detection method of the present invention, two sensor coils are arranged with their back surfaces aligned, a difference between the detection signals is obtained and signal processing is performed to eliminate low-frequency electromagnetic noise and obtain a detection signal. I was able to do it.
According to the low-frequency signal detection method of the present invention, the two sensor coils are arranged with their back surfaces aligned, and the difference between the detection signals is calculated and processed to eliminate low-frequency electromagnetic noise around the production line. The detection signal can be obtained.

FIG. 1 is a conceptual diagram showing an outline of a metal foreign object detection device 1 according to a first embodiment of the present invention. 2A and 2B are diagrams showing a configuration example of the sensor coil. FIG. 2A is a front view of the sensor coil 5a, FIG. 2B is a plan view of FIG. 2A, and FIG. FIG. 3 is a cross-sectional view taken along line AA in FIG. FIG. 3 is a functional block diagram illustrating a configuration example of the control unit 15 of the metal foreign object detection device 1. FIG. 4 is a block diagram showing an outline of the detection circuit 34 according to the first embodiment of this invention. FIG. 5 is a circuit diagram illustrating an example of the detection circuit 34 according to the first embodiment of this invention. FIG. 6 is a conceptual diagram showing an overview of a conventional metal foreign object detection device.

[First Embodiment]
FIG. 1 illustrates an outline of a metal foreign object detection device 1 according to a first embodiment of the present invention. The metal foreign object detection device 1 detects or detects a metal object in the inspection object 2 and notifies it. Although it does not specifically limit as the to-be-inspected object 2, The goods filled into packaging bags, such as a solid, liquid, or gel food, a pharmaceutical, can be illustrated.

[Configuration of Metal Foreign Object Detection Device 1]
The metal foreign object detection device 1 includes a belt 3 that is a conveyance path for conveying an object 2 to be inspected, a sensor unit 4 for detecting foreign objects, a control unit 15 that is a housing that houses a circuit that controls these, and the like. The The belt 3 and the sensor unit 4 are mounted on an upper part of a frame 8 with legs 9 installed on the floor surface. In this example, the control unit 15 is fixed to the lower part of the frame 8. The belt 3 is rotationally driven endlessly by a driving motor 10 provided on one side, and the inspection object 2 is placed thereon and conveyed.

  In this example, the conveyance speed of the belt 3 can be set in a range of about 10 m / min to 50 m / min. The width of the belt 3 can be changed according to the user's request, such as widening or narrowing the production line, but generally, the belt 3 has a 50 cm width specification. The sensor unit 4 is arranged in the conveyance path in which the inspection object 2 is conveyed, in other words, around the center in the length direction of the belt 3. The sensor unit 4 includes a sensor coil for detecting a magnetic body in the inspection object 2. In the first embodiment, the sensor unit 4 includes a first sensor 4 a installed on the lower side of the belt 3 and a second sensor 4 b installed on the upper side of the belt 3.

  The first sensor 4a and the second sensor 4b basically have the same structure, and are composed of a pair of a sensor coil 5 and a sensor coil 6, respectively. The 1st sensor 4a and the 2nd sensor 4b are each provided with the magnet booster 11 of a magnetic field generation means. The sensor coil 5 and the sensor coil 6 have the same structure (details will be described later). A magnetic field generated from a magnetic metal such as a metal object or a metal foreign object in the inspection object 2 changes the surrounding magnetic field.

  Moreover, low-frequency electromagnetic noise may be generated from other devices in the vicinity of the metallic foreign object detection device 1 and reach around the sensor coil 5 and the sensor coil 6. The sensor coil 5 and the sensor coil 6 are for detecting the surrounding low-frequency electromagnetic noise because the surrounding magnetic field is disturbed by the low-frequency electromagnetic noise. The sensor coil 5 and the sensor coil 6 are excited at a low frequency from DC to 10 kHz or less. The sensor coil 5 and the sensor coil 6 output a voltage or current detection signal corresponding to a change in the surrounding magnetic field. The detection signal output from the sensor unit 4 is sent to the control unit 15.

  In the sensor unit 4, a magnet booster 11, which is a magnet for magnetizing the magnetic body in the inspection object 2, is installed on the upstream side of the sensor part 4 when viewed from the flow of the inspection object 2. The magnet booster 11 generates a static magnetic field around it, and the sensor coil 5 and the sensor coil 6 are disposed in the static magnetic field, in other words, in the static magnetic field. Further, the metal foreign object detection device 1 includes a position detection means for detecting that the inspection object 2 has passed the sensor unit 4. As shown in FIG. 1, the position detection unit is composed of a photosensor 7.

  The photosensor 7 is installed adjacent to the sensor unit 4. The inspection object 2 is conveyed by the belt 3 and sequentially passes through the magnet booster 11, the sensor unit 4, and the photosensor 7. The photo sensor 7 transmits a position detection signal for detecting the position of the inspection object 2 to the control unit 15. The control unit 15 analyzes the detection signals received from the sensor unit 4, in other words, the sensor coil 5 and the sensor coil 6, and determines whether or not there is a magnetic substance in the inspection object 2.

  The control unit 15 also uses the position detection signal received from the photosensor 7 for analysis of the detection signal. The control unit 15 notifies the result of this determination (electric signal, warning sound, display, etc.). If the inspection object 2 contains a magnetic material, the control unit 15 outputs an output signal to that effect. This output signal is transmitted from the control unit 15 to a signal tower (not shown) provided in the metal foreign object detection device 1 or a subsequent device. This signal tower emits an alarm signal to the operator by flashing or sounding a buzzer when a shortage is detected.

  As an example of a subsequent apparatus that receives an output signal from the control unit 15, an apparatus for marking the object to be inspected 2, a production line management computer, a factory management control computer, an administrator computer, and the like can be exemplified. The control unit 15 is not necessarily installed in the vicinity of the sensor unit 4 as in this example, and can be installed in any location as long as it can receive and process a detection signal from the sensor unit 4 by wired communication or wireless communication. You may do it. As the communication method of the wired communication and the wireless communication, any known communication standard can be used, and since it is not the gist of the present invention, detailed description thereof is omitted.

[Sensor part]
As shown in FIG. 1, the sensor unit 4 includes one or two magnet boosters 11, a sensor coil 5, and a sensor coil 6. The sensor coil 5 and the sensor coil 6 are the same, and the structure thereof will be described by taking the sensor coil 5 as an example. FIG. 2 illustrates the structure of the sensor coil 5. 2 (a) is a front view of the sensor coil 5, FIG. 2 (b) is a plan view of the sensor coil 5 in FIG. 2 (a), and FIG. 2 (c) is an AA line in FIG. 2 (a). FIG.

  The sensor coil 5 has a configuration in which a coil 21 is wound along a groove portion of an iron core 20 made of a conductive material having an elongated rod shape and an E-shaped cross-sectional structure. The iron core 20 is configured by laminating silicon steel plates or amorphous plates. Alternatively, the iron core 20 may be a ferrite core, a permanent magnet, or the like. When an AC power supply of several kHz is applied to the sensor coil 5, an alternating magnetic field is generated around it. When the magnetic material passes through this, the alternating magnetic field is disturbed, and the current flowing through the sensor coil 5 changes.

  When an electromagnetic wave having a low frequency propagates around the sensor coil 5, the alternating magnetic field is disturbed, and the current flowing through the sensor coil 5 is changed. The disturbance of the alternating magnetic field varies depending on the size and type of the magnetic material and the conveyance speed, but is a change in frequency of about several tens of Hz. From the analysis of the signal detected at the actual site, the disturbance of the alternating magnetic field changes the excitation frequency of the sensor coil 5 at a frequency of about 5 Hz to 10 Hz. The excitation frequency of the sensor coil 5 is low frequency excitation from DC to 10 kHz or less.

  Alternatively, when the magnetic material passes through the static magnetic field when the sensor coil 5 is arranged in the static magnetic field without applying an AC power supply, the static magnetic field is disturbed and changes, and a current flows through the sensor coil 5. . The sensor coil 5 is mainly used for detecting a minute metal of 1 mm or less. Such low-frequency electromagnetic waves are often electromagnetic noise generated from motors used in production lines and the like.

  The sensor coil 5 can simultaneously detect a signal derived from the magnetic metal contained in the inspection object 2 and a signal derived from low-frequency electromagnetic noise. As illustrated in FIG. 2C, the sensor coil 5 has a back surface 22 of the iron core 20 and a surface 23 around which the coil 21 is wound. Normally, the sensor coil 5 is disposed so that the surface of the belt 3 and the surface 23 are parallel to each other with the surface 23 facing the belt 3 through which the detection object 2 flows.

  In the first embodiment, one sensor coil 5 and one sensor coil 6 are combined into one sensor. The sensor coil 5 and the sensor coil 6 are arranged with the back surface 22 aligned, in other words, back to back with each other. The sensor coil 5 is disposed so that the surface 23 on which the coil 21 of the iron core 20 is wound faces the belt 3, and specifically, the surface 23 is parallel to the belt 3. Therefore, the back surface 22 of the iron core 20 of the sensor coil 5 is also disposed on a surface parallel to the belt, but is disposed toward the opposite side of the belt 3.

  The sensor coil 6 is arranged with the back surface 22 of the iron core 20 facing the back surface 22 of the sensor coil 5. The sensor coil 5 detects a metal object by using a non-linear portion in which the magnetic flux density (B) representing the magnetization characteristic and the magnetic field (H) are very small values near zero, among the magnetization (BH) characteristics. I do. The technical idea about this magnetization characteristic and its utilization method is described in detail in Patent Document 1. All or part of the invention described in Patent Document 1 has substantially the same structure and circuit as the present invention, and these constitute the present invention.

  However, the voltages, currents, and frequencies applied to the sensor coils 5 and 6 may be all or partly different from those in Patent Document 1 in the first embodiment of the present invention. Even if the sensor coils 5 and 6 are not specially supplied with current or applied with a voltage, they can detect a metallic foreign object in the magnetic material. For example, the bias voltage of the amplifier is applied to the sensor coils 5 and 6 in advance. As a result, the input bias current of the amplifier flows through the coils of the sensor coils 5 and 6.

  In addition, when the magnetic flux across the sensor coils 5 and 6 changes with the change of the peripheral magnetic field, the current and / or voltage flowing through the sensor coils 5 and 6 change to become an output signal. The static magnetic field generated from the magnet booster 11 corresponds to this peripheral magnetic field. When a stainless steel minute piece approaches the vicinity of the sensor coil 5, the stainless fine piece is magnetized by the action of the static magnetic field to generate a magnetic pole.

  For example, in austenitic stainless steel (SUS304, etc.), a portion having weak magnetism caused by martensite-induced transformation due to plastic deformation such as rupture, crushing, bending, chipping, etc. is magnetized by the action of a static magnetic field. Cause it to occur. The change of the magnetic pole affects the alternating magnetic field, and this is detected by a detection circuit including the sensor coil 5. In particular, since the effect becomes more significant as the inspection object 2 approaches the sensor coil 5, it is possible to reliably detect the metallic foreign object of the minute fragments in the inspection object 2 with the sensor coil 5.

  Examples of the magnetic material mixed as foreign matter in the inspection object 2 include ferromagnetic materials such as iron, nickel, and cobalt, alloys thereof, martensitic stainless steel, and the like. In this example, the sensor coil 6 is installed farther from the inspection object 2 than the sensor coil 5. Further, the surface 23 which is the magnetic field detection surface of the sensor coil 6 is installed so as not to face the belt 3 and the inspection object 2 but to the opposite direction. Therefore, the magnetic field generated from the metallic foreign object of the magnetic substance contained in the inspection object 2 is very small and is hardly detected by the sensor coil 6.

If detected, it is almost negligible. Since the sensor coil 5 and the sensor coil 6 are installed adjacent to each other, almost the same electromagnetic waves generated from other devices around the metal foreign object detection device 1 are detected by the sensor coil 5 and the sensor coil 6.
[Configuration Example of Control Unit 15]
FIG. 3 is a functional block diagram illustrating an outline of a configuration example of the control unit 15 of the metal foreign object detection device 1.

  The control unit 15 includes a bridge circuit 31, an LPF 32, a notch filter 33, a detection circuit 34, a waveform processing unit 35, a digital computer processing unit 36, a signal output unit 37, and the like. The bridge circuit 31 is a circuit that receives signals from the sensor coil 5 and the sensor coil 6. The LPF 32 is a filter circuit that cuts out the high frequency of the signal detected by the bridge circuit 31 and extracts the low frequency. In this example, the LPF 32 passes a signal having a frequency of 1 kHz or less in this example.

  The notch filter 33 is a filter circuit that eliminates a signal in a specific frequency band from the signal that has passed through the LPF 32. In this example, the notch filter 33 erases signals in the 50 Hz and 60 Hz bands. 50 Hz and 60 Hz are commercial electric wiring bands and are often common noise frequencies. The detection circuit 34 processes the detection signal (detection current) from the notch fill 33 and processes it by the digital computer processing unit 36 to output a signal for determining foreign matter.

  The detection circuit 34 deletes a signal derived from low-frequency electromagnetic noise, leaves a signal derived from a foreign object as much as possible, and transmits the signal to the digital computer processing unit 36. The waveform processing unit 35 shapes the waveform of the output signal from the detection circuit 34, converts it into a digital signal, adjusts the reception sensitivity, and outputs the digital signal to the digital computer processing unit 36. The digital computer processing unit 36 receives and processes the digital signal from the waveform processing unit 35 and determines the presence or absence of a magnetic substance contained in the inspection object 2.

  The digital computer processing unit 36 outputs an output signal to the signal output unit 37 when it is determined that the magnetic material is detected. The signal output unit 37 is basically an interface that provides an output signal from the metal foreign object detection device 1 to other devices. The signal output unit 37 transmits the output signal of the digital computer processing unit 36 to an appropriate device or apparatus connected to the metal foreign object detection device 1. For example, it transmits to the signal tower (not shown), the marker apparatus (not shown) for marking to the to-be-inspected object 2, etc.

  Alternatively, the signal output unit 37 has a wireless or wired communication unit, and can transmit the output signal of the digital computer processing unit 36 to an appropriate device or apparatus via a network at a specified address. Although not shown, the digital computer processing unit 36 is a signal processing unit including a memory, a central processing unit (CPU), an input interface, an output interface, and the like. The digital computer processing unit 36 performs signal processing on the signal received from the waveform processing unit 35 or the detection circuit 34 to determine a metal foreign object. This is described in detail in the description of Table 1 below.

  As the digital computer processing unit 36, a general-purpose computer including a storage device such as a ROM and a RAM, a central processing unit (CPU), an input device, and an output device can be used. In this case, the above determination is performed by a dedicated program stored in the storage device. When the digital computer processing unit 36 is an electronic computer, this electronic computer can also serve as the signal output unit 37 and provide its function. The digital computer processing unit 36 can be realized by a circuit means composed of an analog circuit or a digital circuit having the same function, but detailed description thereof is omitted here.

[Overview of detection circuit]
FIG. 4 is a block diagram illustrating an arrangement example of the sensor coil 5 and the sensor coil 6 and an outline of the configuration of the detection circuit 34. In FIG. 4, the source of electromagnetic noise is indicated by reference numeral 16. The noise generation source 16 is a device that is appropriately separated from the metal foreign object detection device 1. In reality, the noise source 16 is separated from the metal foreign object detection device 1 by several tens of centimeters or several meters or more.

  The electromagnetic wave noise generated from the noise source 16 propagates as a spherical surface with the noise source 16 as a center point, as indicated by a broken line 16a. The electromagnetic wave noise reaches the sensor coil 5 and the sensor coil 6 almost at the same time almost at the same magnitude. If the sensor coil 5 and the sensor coil 6 have exactly the same characteristics, the detection signals detected and output by them are also output with the same waveform and amplitude. This is common sense of electronic circuits, but this electromagnetic noise can be eliminated by taking the difference between the detection signals of the sensor coil 5 and the sensor coil 6.

  The difference between the detection signals can be realized by various methods, but one example is shown here. Here, the circuit of FIG. 4 is described by taking the first sensor 4a shown in FIG. 1 as an example, and the second sensor 4b is detected by the same circuit, and therefore the description thereof is omitted. The sensor coil 5 and the sensor coil 6 are arranged on the lower side of the belt 3 with the back surface 22 (see FIG. 2). The inspection object 2 is conveyed by the belt 3 and passes near the detection surface of the sensor coil 5. The sensor coil 5 and the sensor coil 6 each independently detect and output a change in the surrounding magnetic field.

  The output signals of the sensor coil 5 and the sensor coil 6 are rectified by rectifiers 40 and 41, respectively. The output signal of the rectifier 41 is inverted by the inverter 42. The sensor coil 5 and the sensor coil 6 detect the same electromagnetic noise derived from the noise source 16. By this phase inversion, the electromagnetic wave noise signal detected by the sensor coil 6 is in the opposite phase to the electromagnetic wave noise signal detected by the sensor coil 5. The adder 43 adds the output signal of the rectifier 40 and the output signal of the inverter 42 and outputs the result.

  By this addition processing, the electromagnetic wave noise derived from the noise source 16 is eliminated. Therefore, from the adder 43, only the foreign object detection signal derived from the inspection object 2 remains and is output. In principle, the signal output from the adder 43 can determine whether or not there is a metallic foreign object in the inspection object 2. That is, when a signal equal to or greater than a predetermined threshold is output from the adder 43, it is determined that there is a metal foreign object in the inspection object 2.

[Example of detection circuit]
FIG. 5 is a circuit diagram showing an example of the detection circuit 34 more specifically. The sensor coil 5 and the sensor coil 6 are arranged so that the back surfaces 22 (see FIG. 2) are aligned with each other, and function as one sensor. The circuit example of FIG. 5 is a circuit more specifically showing the circuit configuration of FIG. In the circuit of FIG. 5, detection signals from the sensor coil 5 and the sensor coil 6 are amplified by an amplifier 52.

  Since the detection signals of the sensor coil 5 and the sensor coil 6 are weak signals, they are amplified for subsequent signal processing. In this example, the amplifier 52 amplifies up to 120 dB. The amplified detection signal is rectified by the rectifiers 51 and 54. The rectifiers 51 and 54 are diodes. Only the detection signal of the sensor coil 6 is transferred and adjusted in phase by the phase transfer circuit 53. The detection signal of the sensor coil 6 can be phase-adjusted by the phase transfer circuit 53 either before or after rectification by the rectifier 54, and may be either. The phase transfer circuit 53 is not limited, but an all-pass filter or the like may be used.

  The signal rectified by the detection signal of the sensor coil 5 is output from the output terminal 62 as it is. This output signal is input as a second input to the digital computer processing unit 36 at the subsequent stage. The signal rectified by the detection signal of the sensor coil 5 is temporarily buffered by the buffer 57. A signal rectified by the detection signal of the sensor coil 6 is input to the peak hold circuit 55. The signal rectified by the detection signal of the sensor coil 6 is temporarily buffered by the buffer 58.

  The signal temporarily held in the buffer 58 is inverted in phase by the inverter 59 and input to the adder 60. Similarly, the signal temporarily held in the buffer 57 is also input to the adder 60. The adder 60 adds these input signals and outputs them. Since the detection signal of the sensor coil 6 is phase-inverted and added to the detection signal of the sensor coil 5, the same signal detected simultaneously by the sensor coil 5 and the sensor coil 6 is deleted. In other words, the electromagnetic noise signal is erased to zero.

  The output of the adder 60 is output from an output terminal 61, and this output signal is input as a first input to the digital computer processing unit 36 at the subsequent stage. When there is electromagnetic wave noise, when the inspection object 2 mixed with metal foreign matter is conveyed, a signal in which the electromagnetic noise signal and foreign matter signal are mixed is output as the output signal of the sensor coil 5. The first input at this time is a signal derived from the foreign substance signal, and the foreign substance signal is output.

  If the sensor coil, IC circuit, transistor, resistor, capacitor, etc. of the components of the electronic circuit are linear, the electromagnetic noise can be eliminated smoothly in principle with the circuit configuration shown in FIGS. 4 and 5 described above. is there. However, realistically, since all the elements constituting the electronic circuit and the magnetic field of electromagnetic noise are non-linear, the noise does not disappear smoothly. For example, even if the signal waveforms of the detection signals detected by the sensor coils 5 and 6 have the same shape, the amplitudes are slightly different. Further, even if the amplitude is the same, the signal waveform shape is partially different and is non-linear.

  Further, a physical phase difference is generated in the detection signals of the sensor coil 5 and the sensor coil 6 depending on the frequency of electromagnetic noise and the position and orientation of the generation source. These problems are solved by using the phase transfer circuit 53, the peak hold circuit 55, the reset circuit, and the like. Specifically, the waveform of the electromagnetic wave noise of the sensor coil 6 is shifted (advanced or delayed), the waveform is shaped, output from the terminal 63, and input to the digital computer processing unit 36 at the subsequent stage as the third input.

  The peak hold circuit 55 is a circuit for storing and holding the peak value of the electromagnetic noise signal detected by the sensor coil 6, and is a circuit for absorbing the amplitude fluctuation of the electromagnetic noise. It is necessary to reset the peak value held by the peak hold circuit 55 when the electromagnetic wave noise disappears or when the inspection object 2 is detected. In this example, the photosensor 7 (see FIG. 1) is used to detect the timing at which the inspection object 2 passes through the sensor coil 5.

  Therefore, the peak value of the peak hold circuit 55 is reset by the detection signal of the photosensor 7. That is, when the inspection object 2 is detected, the peak hold circuit 55 holds the peak value of the electromagnetic noise, and the photo sensor 7 indicates the time when the inspection object 2 has passed through the sensor coil 5. The peak value of the peak hold circuit 55 is reset using the falling signal. This reset signal is input from the transistor 56 to the peak hold circuit 55.

[Detection algorithm]
Using these first to third input signals, the digital computer processing unit 36 performs signal analysis to determine foreign matter. An example of foreign object determination in the digital computer processing unit 36 is shown in Table 1 below. The first column of Table 1 shows the type of input signal of the digital computer processing unit 36, and the last line is the determination result.

Columns 2 to 9 in Table 1 show combinations of input signals of the digital computer processing unit 36. Since there are three types of input signals from the first input to the third input, the number of combinations is eight. In the table, when there is an input signal, it is indicated by a circle (“◯”), and when there is no input signal, it is indicated by a minus (“−”). In the determination of foreign matter detection in the last row of Table 1, “present” is determined that there is a metal foreign matter, and “no” is determined that there is no metal foreign matter.

  In the normal inspection of the inspection object 2, detection and signal analysis are performed only by the output of the sensor coil 5 indicated by the second input. Metal foreign matter detection is performed when there is a signal only at the second input and when there is a signal at the second input during the first input or the third input. The first combination indicates a normal good product. In this case, neither the sensor coil 5 nor the sensor coil 6 detects the signal. Normal metal foreign matter determination is the fifth combination. In this case, the signal is detected only by the sensor coil 5.

  When normal electromagnetic noise is generated, the sixth or seventh combination is used. As shown in the case of the eighth combination in the last column, if there is a signal in all of the first input to the third input, it is difficult to determine the metal foreign object. In the case of the eighth combination, the magnitude (energy) of the first input signal exceeds or does not exceed the normal sensitivity level, and in addition to this, a metal foreign object is detected from the phase relationship of each signal, or the object to be inspected It is determined whether the object 2 is a non-defective product.

[Magnet booster]
The magnet booster 11 of the metal foreign object detection device 1 according to the first embodiment of the present invention provides a magnetic field to the sensor coil 5 and the sensor coil 6 to magnetize the magnetic body in the inspection object 2 or to enhance the magnetization. Is to do. When the sensor coil 5 and the sensor coil 6 are excited, the magnet booster 11 is used to magnetize or enhance the magnetization of the magnetic body in the inspection object 2.

  In this case, the magnet booster 11 can be arranged away from the sensor coil 5 and the sensor coil 6. Moreover, when the sensor coil 5 and the sensor coil 6 are excited, the magnet booster 11 can simultaneously provide a static magnetic field to the sensor coil 5 and the sensor coil 6. In this example, although not limited, the sensor coil 5 and the sensor coil 6 are not illustrated, and the magnetic booster 11 is provided with a static magnetic field.

  The magnet booster 11 is made of a permanent magnet. The magnet booster 11 is installed in front of the sensor unit 4 (supply side) in the conveying direction of the inspection object 2. When viewed in the conveyance direction of the inspection object 2, the inspection object 2 is conveyed through the belt 3, passes through the magnet booster 11, and then passes through the sensor coil 5. The magnet booster 12 is composed of one permanent magnet or two or more permanent magnets. In the first embodiment, as illustrated in FIG. 1, the magnet booster 11 includes a magnet 12 fixed to the lower side of the belt 3 and a magnet 13 fixed to the upper side of the belt 3.

  In particular, by arranging the magnet 12 fixed to the lower side of the belt 3, the inspection object 2 can pass with the magnet 12 closest. The magnet 12 or the magnet 13 is arranged with the N magnetic pole or the S pole facing the object to be inspected 2. The direction of the magnetic pole is optimized according to the type, size, etc. of the inspection object 2 and is not limited to the above arrangement. The magnets 12 and 13 are composed of a plurality of magnet elements made of neodymium magnets.

  Thus, by using a strong magnet such as a neodymium magnet, the range in which the magnetic field of the permanent magnet affects the magnetic material can be greatly expanded, and the detection distance of the sensor coil 5 can be greatly extended. For this reason, according to the shape and width of the inspection object 2, the magnetizing force of the magnets 12 and 13 and the gap with the belt 3 through which the inspection object 2 passes can be adjusted.

[Others]
The sensor unit 4 includes sensor coils 5 and 6 disposed on the upper side of the belt 3 and sensor coils 5 and 6 disposed on the lower side of the belt 3. This is configured in this way in order to detect the upper and lower parts of the inspection object 2 with high sensitivity. Metal foreign matter in the inspection object 2 can be detected only on the upper side or the lower side of the belt 3. In the metal foreign object detection device 1 of the first embodiment, even if the object to be inspected 2 is an aluminum alloy packaged or inserted into an aluminum alloy bag, the magnetic substance therein can be detected well.

  Thus, electromagnetic noise can be eliminated by arranging the sensor coils 5 and 6 back to back. Therefore, as shown in FIG. 6, conventionally, the detection sensors 70 and 71 detect two times, but only the sensor unit 4 can detect a metal foreign object with a single detection. Therefore, only one pair of sensor units 4 is provided, and the conventional limitation on the conveyance speed that is restricted by the detection time is eliminated. As shown in FIG. 1, the sensor unit 4 includes a first sensor 4 a and a second sensor 4 b, but either one of them may be used.

  The sensor coils 5 and 6 are installed on the upper side or the lower side of the belt 3, but may be on the side of the belt 3. However, if the sensor coil 5 is installed under the belt 3 with a slight gap from the belt 3, the sensor coil 5 can be disposed closest to the object to be inspected 2 and the detection sensitivity is improved accordingly. In the present embodiment, the sensor unit 4 does not show a magnetic shield, but the sensor coils 5 and 6 house the sensor unit 4 in a metal casing or a casing made of a material having a magnetic shielding effect. Can be shielded from an external magnetic field.

  In this example, the photosensor 7 uses an optical sensor, but any known sensor can be used as long as it can detect the conveyance timing of the inspection object 2 and the time when the sensor coil 5 is added. Available. The sensor coil 5 can be arranged so as to have a right angle or a predetermined angle with respect to the conveyance direction of the inspection object 2. In the case of a right angle, the inspection object 2 that has passed through the sensor coil 5 can be easily detected by the photosensor 7, and therefore a right angle is preferable. In this example, the sensor coil 6 is disposed so that its back matches the back of the sensor coil 5.

  If the detection surface of the sensor coil 6 is not directed to the inspection object 2 and is installed in the opposite direction, the signal due to the magnetic substance in the inspection object 2 is most difficult to detect. However, since the role of the sensor coil 6 is to detect electromagnetic noise with the same sensitivity as the sensor coil 5, the sensor coil 6 can be optimized according to the installation location of the metal foreign object detection device 1 and the surrounding situation. For example, the sensor coil 5 and the sensor coil 6 may be installed at a predetermined angle.

  The present invention is preferably used in the field of detecting a magnetic field or a low-frequency electromagnetic wave in an environment with low-frequency electromagnetic noise. Further, the present invention may be used in the field of manufacturing pharmaceuticals, cosmetics, and foodstuffs, and in the field of detecting metallic foreign objects in products while eliminating low-frequency electromagnetic noise.

DESCRIPTION OF SYMBOLS 1 ... Metal foreign material detection apparatus 2 ... Test object 3 Belt 4 ... Sensor part 4a ... 1st sensor 4b ... 2nd sensor 5, 6 ... Sensor coil 7 ... Photo sensor 8 ... Frame 9 ... Leg 10 ... Drive motor 11 ... Magnet booster 15 ... Control unit 16 ... Noise source 20 ... Iron core 21 ... Coil 22 ... Back side (of sensor coil) 23 ... Surface (of sensor coil) 31 ... Bridge circuit 32 ... LPF
33 ... Notch filter 34 ... Detection circuit 35 ... Waveform processing unit 36 ... Digital computer processing unit 37 ... Signal output unit 40, 41 ... Rectifier 42, 59 ... Inverter 43 ... Adder 50, 52 ... Amplifier 51, 54 ... Rectifier 53 ... Phase transfer device 55: Peak hold circuit 56 ... Transistor 57, 58 ... Buffer 60 ... Adder 61, 62, 63 ... Terminal

Claims (9)

A detection method of a low frequency signal for obtaining a detection signal by removing a noise signal from a detection signal in which a magnetic field or an electromagnetic wave is detected by a detection means,
The detecting means includes a magnetic field generating means for generating a magnetic field, a first sensor coil arranged in the magnetic field, and a second sensor arranged in the magnetic field in the vicinity of the first sensor coil. Consisting of coils,
Each of the first sensor coil and the second sensor coil is for detecting a change in the surrounding magnetic field or a low-frequency electromagnetic wave, and has a configuration in which a conductor is wound around a core, The detection is performed using a non-linear portion in which the magnetic flux density (B) and the magnetic field (H) of the magnetization (BH) characteristic are very small values near 0,
A difference signal indicating a difference between the first detection signal detected by the first sensor coil and the second detection signal detected by the second sensor coil is obtained by the signal processing means, and the low frequency noise signal is removed.
When the difference signal is greater than or equal to a predetermined amplitude value, the signal processing means outputs the detection signal indicating the change magnetic field in which the magnetic field changes or the low frequency electromagnetic wave that is not derived from the low frequency noise signal. A method for detecting a low-frequency signal. The method of detecting a low frequency signal according to claim 1,
The surface of the first sensor coil, which is a surface of the first sensor coil wound around the core, is disposed toward the source of the change magnetic field or the low frequency electromagnetic wave, and the change magnetic field or the low frequency electromagnetic wave. Detecting the above,
The surface of the second sensor coil, which is the surface of the second sensor coil wound around the core, is arranged in a direction opposite to the surface of the first sensor coil or toward a noise signal generation source that is a generation source of the noise signal. And
The method of detecting a low frequency signal, wherein the noise signal is detected by the first sensor coil and the second sensor coil. The method of detecting a low frequency signal according to claim 2,
The method of detecting a low frequency signal, wherein the magnetic field is a magnetic field formed by exciting the first sensor coil and the second sensor coil and / or formed by a magnet booster. The method for detecting a low frequency signal according to claim 1 selected from claims 1 to 3.
The method of detecting a low frequency signal, wherein the intensity of the noise signal is greater than the magnetic field intensity of the changing magnetic field or the low frequency electromagnetic wave. The method for detecting a low frequency signal according to claim 1 selected from claims 1 to 3.
The change magnetic field is generated by moving a magnetic field generation source in the magnetic field of a static magnetic field. In the detection method of the low frequency signal of Claim 5,
The method of detecting a low frequency signal, wherein the magnetic field generation source is a minute magnetic material. The method for detecting a low frequency signal according to claim 1 selected from claims 1 to 3.
The low frequency noise signal is an electromagnetic noise or a noise magnetic field generated from an electromagnetic noise source selected from an electric line, an electric device, an electronic device, and a moving or rotating permanent magnet. Detection method. The method for detecting a low frequency signal according to claim 1 selected from claims 1 to 3.
Rectifying the first detection signal and the second detection signal;
A method of detecting a low frequency signal, wherein the rectified second detection signal is phase-inverted and added to the rectified first detection signal to obtain the difference. The method of detecting a low frequency signal according to claim 8,
The phase of the detected low-frequency noise signal, or the phase of the first detection signal and the second detection signal according to the arrangement of the first sensor coil and the second sensor coil is the second detection signal or the A method for detecting a low-frequency signal, comprising adjusting the first detection signal by phase-shifting with a phase-transfer circuit.

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